This invention relates to materials for and methods of producing matching layers for ultrasound transducers. In particular, this invention relates to matching layers for ultrasound transducers that have a gradient in impedance value between the impedance of the transducer material and that of the target.
Ultrasound prober typically are made up of the transducer piezoelectric ceramic elements sandwiched between the backing or damping layer and a set of matching layers. The backing layers prevents the backward emitted sound waves to echo and ring back into the transducer for detection. The matching layer or layers provide the required acoustic impedance gradient for the acoustic energy from the transducer to smoothly penetrate the body tissue and for the reflected acoustic waves (the returning echo) to smoothly return to the transducer for detection. Without the matching layers, the large impedance difference between the acoustic source (about 33 Mrayls) and the target (about 1.5 Mrayls) would result in loss of transmission and receipt of acoustic energy of up to 90 percent at the interface between the source and the target. Typically, the matching layers are designed to have specific impedance values (e.g., about 15 and 3 Mrayls) and are attached to the transducer. The stepwise reduction of the impedance at the interfaces minimizes the loss in the transmission and receipt of the returning acoustic signals. A matching layer structure with a gradient of impedance across its thickness from hat of the transducer elements (about 33 Mrayls) to that of the body tissue (about 1.5 Mrayls) is the ideal structure for zero loss of signal in the absence of any attenuation of the signal by the matching layer itself. Such a layer would also enhance the fractional bandwidth from a typical 70 percent to 90 percent or more. Such a wider bandwidth allows the transducer to be used selectively in the burst excitation mode at more than one frequency with the accompanying freedom to choose higher resolution of the image details or longer penetration of the beam energy. The optimal thickness for each of the matching layers is one-fourth of the wavelength of the central operating frequency of the transducer elements. Thus, the manufacture of the matching layers can be a challenge because of such a small desired thickness. Matching layers thicker than one-quarter wavelength may be used, but they increase the attenuation of the ultrasound intensity with the attendant reduced performance.
A matching layer having an impedance gradient has been proposed in U.S. Pat. No. 5,974,884. A first material having first impedance equal to or lower than the impedance of the transducer material is formed in a matrix of tapered cone-shaped elements. A second material having second impedance equal to or greater than the impedance of the target living tissue is used to fill the interstices of the matrix and form the finished matching layer. Due to the cone shaped of the first material, the impedance of the matching layer decreases continuously from the surface where the bases of the cones reside to the opposite surface where the cone vertices reside. However, the manufacture of such a matching layer having a thickness on the order of one-quarter wavelength using this method is tedious and could be costly.
Therefore, it is desirable to provide ultrasound probe matching layers that are simple to manufacture and that still have a gradient in impedance or an impedance value varying from one surface of the matching layer to the other surface.